The present invention is directed to the focusing of ink drops on a spaced apart substrate, and more particularly to lateral focus of aqueous ink drops onto a substrate through the implementation of electric fields for use in acoustic ink printing.
Various fluid application technologies, such as printing technologies, are being developed. One such technology uses focused acoustic energy to emit droplets of a marking material from a printhead onto a recording medium. This application is called acoustic ink printing (AIP) and is described in a number of U.S. patents, including U.S. Pat. Nos. 4,308,547, 4,697,195, 5,028,937 and 5,087,931, the disclosures of which are incorporated herein by reference.
Acoustic ink printheads typically include a plurality of droplet emitters, each of which projects a converging acoustic beam into a pool of liquid. The angular convergence of this beam is selected so that the beam comes to focus at or near the free surface of the liquid, that is, at the liquid/air interface. Printing is performed by modulating the radiation pressure that the beam of each emitter exerts against the free surface of the liquid, to selectively emit droplets of liquid from the free surface.
More particularly, modulating the radiation pressure of each beam causes the radiation pressure to make brief, controlled excursions to a sufficiently high pressure level to overcome the restraining force of the surface tension at the free surface. Individual droplets of liquid are emitted from the free surface of the pool of liquid on command, with sufficient velocity to deposit them on a nearby recording medium.
Ideally, all of the actuators in a printhead produce drops directed toward the print substrate in a direction perpendicular to the print substrate. In practice, however, some drops are not directed exactly perpendicular to the print substrate. The drops which deviate from the desired trajectory are undesirable since the misdirected drops impact the print substrate at a point not anticipated by the print controller. Therefore, misdirected drops affect the quality of the printed image by impacting the print substrate in unwanted positions.
U.S. Pat. Nos. 4,386,358 and 4,379,301 to Fischbeck, which are commonly assigned and incorporated herein by reference, disclose a method for electrostatically deflecting electrically charged ink drops emitted from an ink jet printhead. Charges placed on electrodes on the printhead disclosed by Fischbeck are controlled to steer the charged ink drops in desired directions to compensate for known printhead movement. By electrostatically steering the charged ink drops, the method disclosed in Fischbeck compensates for ink drop misdirection caused by the known printhead movement when the ink drop is emitted.
However, the electrostatic deflection method disclosed by Fischbeck does not compensate for unpredictable environmental factors which can affect ink drop trajectories. Such environmental factors include air currents and temperature gradients between the printhead and the print substrate. In acoustic ink jet printheads, unpredictable variations in the dynamics of ink drop creation also detrimentally affect ink drop trajectories. Some of the variations in ink drop creation are caused by aberrations in the lithography of Fresnel lens which are in some embodiments used to focus the acoustic wave used to create the ink drops.
U.S. patent application Ser. No. 08/480,977 entitled "Electric-Field Manipulation of Emitted Ink Drops in Printing", which is commonly assigned, and is hereby incorporated by reference, discloses the use of an electric field to reduce droplet misdirectionality, by inducing a charge on a drop as it breaks off from the bulk of the fluid. The charged drop is then accelerated into the paper, by holding the paper at a relatively large potential (this same potential may be used to induce the charge on the drop). The application teaches selectively deflecting the ink drops slightly to enhance the resolution of the image produced by a given printhead configuration. The ink jet actuators form and impart an initial velocity on the ink drops. The charged ink drops are then steered by electrodes such that the drops alternately impact upon the print medium at positions slightly offset from positions directly opposite the apertures of the printhead.
This approach, though useful, has drawbacks. It requires large voltages, of the order of 1 to 2 kV. Also, it will suffer from many of the same imaging artifacts as occur in ionographic printing, where because charge is deposited onto the printing substrate, there is print-dependent interaction of the accelerating field with the charged drop. That is, as drops are accumulated on the paper, so is their charge. If this charge is not removed quickly enough, it will produce a print-dependent potential at the paper surface, which will interfere with the acceleration of subsequent drops. Finally, the acceleration expected for drops under typical print conditions is only large enough to reduce the misplacement of drops by some 50% at the paper surface, so that the correction of the misdirection, while significant, is not complete.
U.S. patent application Ser. No. 08/721,290 entitled "Method and Apparatus for Moving Ink Drops Using an Electric Field", which is commonly assigned, and is hereby incorporated by reference, discloses using an electric field to charge and impart a force onto ink drops to control for motion of the ink drops, including biasing the print support medium with a voltage source.